An example of conventional IC card including a power supply switching circuit is shown in FIG. 1. In FIG. 1, an IC card 10 includes therein semiconductor devices 1 and 2, such as IC devices, including memories and microcomputers, an internal power supply 3, and a power supply switching circuit 4 which will be described later. The IC card 10 includes a connector 6 for connecting the IC card to an external device (not shown). The connector 6 is connected to an external-power-supply line 7 through which external power is supplied to the IC card, a common ground line 8, an address line 9 connected in common to the semiconductor devices 1 and 2, a control line 11 also connected in common to the semiconductor devices 1 and 2, a data line 12 for transferring data between memories within the semiconductor device 1 and the external device, and a data line 13 for transferring data between memories within the semiconductor device 2 and the external device, which are all within the IC card 10.
The external-power-supply line 7 is connected to the anode of a first diode 14 which acts as a switching element of the power supply switching circuit 4. The internal power supply 3 is connected via a resistor 15 to the anode of a second diode 16 which also acts as a switching element of the power supply switching circuit 4. The cathodes of the diodes 14 and 16 are connected to a common power supply line 18 for coupling operating voltages to the semiconductor devices 1 and 2. The control line 11 is connected via a pull-up resistor 19 to the commonly connected cathodes of the diodes 14 and 16 of the power supply switching circuit 4. The commonly connected cathodes of the diodes 14 and 16 receive a voltage V.sub.cc.
When the connector 6 is not connected to the external device, the IC card 10 is operated from a voltage supplied by the internal power supply 3 of, for example, 3 V. Specifically, when the connector 6 is not connected to the external device, the diode 16 is conductive so that an operating voltage is supplied to the semiconductor devices 1 and 2 from the internal power supply 3 via the resistor 15, the conducting diode 16 and the power supply line 18. Thus, the IC card 10 is placed in a stand-by mode.
In contrast, when the connector 6 is connected to the external device, an external voltage of, for example, 5 V is applied from an external power supply through the connector 6 to the external-power-supply line 7. This turns the diodes 14 and 16 in the power supply switching circuit 4 on and off, respectively, so that an operating voltage is supplied from the external-power-supply line 7 to the semiconductor devices 1 and 2 via the diode 14 and the power supply line 18 to condition the memories in the semiconductor devices 1 and 2 for normal operation. When an address signal is applied to the address line 9 and the control line 11 is made active, data can be written into or read from the memories through the data lines 12 and 13.
Current flowing when the above-described conventional IC card operates from the internal power supply 3, i.e. standby current I.sub.s, is several microamperes, and, therefore, as is understood from the diode V-I characteristic curve of FIG. 2, a voltage drop V.sub.s across the diode 16 of the power supply switching circuit 4 is very small and, accordingly, has no adverse influence on the operation of the IC card. In contrast, when the IC card 10 is operated from the external power supply, current flowing through each of the semiconductor devices 1 and 2 is 50-100 mA, which is significantly larger than the standby current. As shown in FIG. 2, a voltage drop V.sub.2 across the diode 14 when current 2I for operating the semiconductor devices 1 and 2 flows is, for example, 0.7 V or so, and, therefore, operating voltages applied to the respective semiconductor devices 1 and 2 decrease. This voltage decrease causes degradation of operational characteristics of the semiconductor devices 1 and 2. For instance, the time required for accessing the memories may become longer, proper timing for writing into or reading from the memories may be lost, etc. In the worst case, the voltages applied to the respective semiconductor devices 1 and 2 may be outside the proper range of operating voltages, and the IC card may become inoperative.
The object of the present invention is to provide an IC card which is free of the above-stated problem and in which the voltage drop across a diode of a power supply switching circuit is kept small so that the IC card can always operate properly.